sodium salt of unknown aromatic + soluble impurities + insoluble impurities total insoluble impurities dissolve in water, filter aq. sol n of unk. sodium salt + sol. impurities account for all masses; calculate %recovery C361/361 Week 3 Lecture What decolorize, filter Affects pk a? Identify unknown Derivatization to Amides colored dye(s) mass: ~ 20 mg aq. sol n of unk. sodium salt + sol. impurities conc. ; filter mp, MW, pk a, derivative mp water-soluble imp. crude unk. recrystallize pure unk. melting point ( mp )
sodium salt of unknown aromatic + soluble impurities + insoluble impurities total dissolve in water, filter account for all masses; calculate %recovery insoluble impurities aq. sol n of unk. sodium salt + sol. impurities decolorize, filter Identify unknown colored dye(s) mass: ~ 20 mg aq. sol n of unk. sodium salt + sol. impurities conc. ; filter mp, MW, pk a, derivative mp water-soluble imp. crude unk. recrystallize pure unk. melting point ( mp )
Affect of solvent and temperature on pk a + 2 + 3 benzoic pk a = 4.19 K a = PhC 2 3 + PhC 2 pk a = log 10 K a must be fully dissolved to determine pka, but, many have low to modest solubility in pure water; how could be fully dissolved? raising temperature helps to dissolve, but pka is defined at standard state, different temperature will change pka value; solution must be at rt when conducting potentiometric titration addition of ethanol raises pka; need to approximately correct for likely effect of any added Et (see p. 16 of lab manual for correction data) (note: adding ethanol will not affect outcome of end-point titration)
Affect of substituents on pk a benzoic 2-hydroxybenzoic 3-hydroxybenzoic 4-hydroxybenzoic pk a 4.19 2.97 4.08 4.61 group in para position: due to resonance, electron density from -atom lonepair is delocalized over aromatic ring leading to increased negative charge character near carboxylate anion results in destabilization of conjugate base (lowers ity; raises pka)
Affect of substituents on pk a benzoic 2-hydroxybenzoic 3-hydroxybenzoic 4-hydroxybenzoic pk a 4.19 2.97 4.08 4.61 group in meta position: resonance does not push significant charge density to site of carboxylate anion δ + δ but, electronegative character of -atom leads to C sigma-bond polarization carboxylate anion stabilized by inductive effect (a localized effect)
Affect of substituents on pk a benzoic 2-hydroxybenzoic 3-hydroxybenzoic 4-hydroxybenzoic pk a 4.19 2.97 4.08 4.61 group in ortho position: as in para case, resonance again pushes significant charge density to site of carboxylate anion inductive effect is also maximized (close proximity) intramolecular hydrogenbond provides additional stabilization to carboxylate anion
Synthesis of a derivative to aid identification of unknown aromatic mpt 146 C mol. wt. 167.12 pk a 2.22 'amide' mpt 176 C 'anilide' mpt 155 C N 2 148 C 201.02 2.85 155 C 141 C Br R N 2 N 3 R N 2 N 'amide' a 1 amide ammonia 'anilide' a 2 amide of aniline aniline (aminobenzene)
Amide Formation Strategy: Interconversion of Carboxylic Acid Derivatives N 3 + N 3 N 2 2 N 4 activation process ic & poor leaving group N 3 N 4 X N 3 X aprotic & good leaving group X N 3 N 3 X
Conversion of a Carboxylic Acid to an Acyl Chloride and Further to Amide S 2 + + S 2 thionyl chloride N 1 amide 3 N 2 aromatic N dimethyl formamide chloride (acyl halide) ammonia + N + 4 ammonium chloride Ph N S dimethyl formamide (DMF) catalyzes the formation of the acyl chloride N Ph N S DMF is not consumed in the reaction, but it accelerates the rate of the transformation N Ph S 2
Tasks for Week 3: Finish Large Batch Recrystallization, End-point Titration polar non-ionic compound sparing solubility in 2 n of unk. sodium salt mpurities conc. ; filter mp, MW, pk a, derivative mp R + related impurities precipitate (solid) crude unk. melting point ( mp ) recrystallize pure unk. Wednesday Friday finish large batch recrystallization, dry solid, calculate %recovery, obtain accurate mp; TIME ALLWING, begin end-point titration (finish large batch recrystallization), end-point titration (Friday) TIME ALLWING begin potentiometric titration